The rapid advancement of multimedia applications requires a system that stores more information more compactly, and is easier to use and handle, and possesses superior performance in every respect. The digital video disk (DVD) technology offers that advantage, and its application field extends from movie entertainment to computer data storage. In fact, the DVD has a tremendous role to play wherever digital technology rules--that is, everywhere big capacity is a must.
DVD is alternatively called digital versatile disk. It includes products and software that will be built in conformance with a specification being developed by a consortium of the largest computer, consumer electronics, and entertainment companies. The intention is to create a range of compatible products based on a new generation of the Compact Disk format which provides increased storage capacity and performance, especially for video and multimedia applications.
A compact disk (CD) can only store 5 minutes worth of analog image information, and a larger size laser disk of 11.8 inch can store 60 minutes of images. The movie and computer industries both want a small disc with CD compatibility, because the CD, which is the global standard for music and computer software, is increasingly packaged on CD ROMs. People are familiar with these convenient, space-saving disks. Also, both industries want a disk with enough single-side capacity to handle the general run of tasks. There has to be enough capacity for interruption-free movie viewing and for fast, reliable access and retrieval of computer data. The new DVD standard allows a single side DVD to hold 4.7 gigabytes of information, enough to store 135 minutes of high quality movie. The DVD also offers great advantages to users of computers.
As the industry introduces new digital technologies, an increasingly important consideration is compatibility with previous formats. Great effort has gone into making the DVD system, such as DVD-ROM, backward compatible. That means you will be able to play the existing CD format on a DVD system. However, there exist quite a few physical differences between a DVD system and a CD system. For instance, a DVD format disk has a pit length of 0.4 .mu.m and a track pitch of 0.74 .mu.m, almost half that of a CD format disk. In addition, the DVD substrate is 0.6 mm, in comparison to a CD substrate of 1.2 mm. The DVD laser wavelength is either 635 nm or 650 nm, and the DVD numerical aperture (NA) of the focal lens is 0.6, while the CD laser wavelength is 780 nm, and the CD NA of the focal lens is 0.45. These different parameters between DVD and CD lead to difference between a DVD pickup module and a CD pickup module.
Several approaches have been proposed to solve the DVD backward compatibility issue. Among the candidate solutions are an integrated two-lens switching system which consists of two lenses, one for CD, and the other for DVD, and a dual-focus pickup system in which a single lens is used to achieve focus for both CD and DVD. With the integrated two-lens switching system, two lenses, one for CD and the other for DVD, are switched by rotating the lenses horizontally to read signals for each disk. With the dual-focus pickup system, a single lens can achieve focus for both CD and DVD without being realigned, and can read the signals, due to the adoption of a hologram lens.
The two lens system and the hologram lens system are complicated and difficult to make. Another technique is to use an LCD shutter to alter the characteristics of the laser used in reading disks. The LCD shutter pickup is less complicated than the two-lens and hologram formats, resulting in a smaller, more reliable pickup. The pickup is built using a 635 nm red laser. The LCD shutter changes the numerical aperture of the lens, producing a narrower beam, resulting in a longer focal distance of 1.2 mm into CDs, or a wider beam, resulting in a shorter focal distance of 0.6 mm into DVDs. This allows reading the two different formats with one pickup.
Although the LCD shutter pickup is an improvement to either the two lens pickup system or the hologram lens pickup system, it is not an efficient use of the laser output power because the shutter blocks a significant amount of the laser power to narrow the beam width. Power efficiency is very important to laptop computer applications. Also, in the LCD shutter pickup system extra circuitry is required to control the LCD shutter.
Thus it is highly desirable and an object of the present invention to provide for a low cost, compact optical pickup module, that has included therein a vertical cavity surface emitting laser (VCSEL) of 780 nm and a visible edge emitting laser, or another visible VCSEL, which module utilizes low cost materials, and is simple to fabricate.
It is another purpose of the present invention to provide for a new and improved low cost optical pickup that can read both CD and DVD, and is energy efficient.